Rotting trees in forests are a very significant source of food and habitat for many organisms, including snails. In one of the first papers I published on land snails1, I had a sentence about the importance of not removing the fallen trees from wooded areas. I had been influenced not only by my own observations, but also by a USDA publication on the subject2.
An organism that lives in or on rotting organic matter is called a saprophyte. Many saprophytes can survive nowhere but on dead trees. I don't know if there are any snail species that are strictly saprophytic, but many do hang around rotting tree trunks, because that is where they can find food (the fungi that grow on dead wood), moisture and shelter. However, in most cases the species that can be collected from rotting trees have also been observed eating live plants in the wild and will eat fresh vegetables in captivity.
In most wooded parks that I have visited in the U.S. I have seen plenty of rotting trees. I hadn't realized that this wasn't generally the case in European forests until I read a recent publication by the World Wildlife Fund3. According to the WWF, “Average forests in Europe have less than 5 per cent of the deadwood expected in natural conditions.” Perhaps because of this shortage of deadwood, WWF also reports that the species associated with deadwood is the largest single group of threatened species in Europe.
Even if you can't do anything else, you may consider keeping a small log or two in a secluded spot in your yard to help some saprophytes survive4. Many people seem to think that rotting wood is an ugly sight. That is not necessarily the case. In fact, I consider many saprophytic fungi rather pretty on their own terms. They are also good subjects for photography, because they don't move much.
1. Örstan, A. 1999. Land Snails of Black Hill Regional Park, Montgomery County, Maryland. Maryland Naturalist, 43(3-4):20-24. pdf (from Aydin’s Library)
2. Maser, C., & Trappe, J.M. 1984. The seen and unseen world of the fallen tree. General Technical Report PNW-164. U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, Oregon.
3. Dudley, N. & Vallauri, D. 2004. WWF Report. Deadwood - living forests. The importance of veteran trees and deadwood to biodiversity. pdf (from WWF)
4. Keep such logs away from your house to avoid potential termite infestations.